Not exactly, the majors all have options to get a better or "Premium Seat" in some cases it's baked into the class of the ticket.

Delta has a Basic Economy with no seat assignment which is $15-20 less than their Main Cabin which does have limited seat assignment, better seats are available for purchase or if you have status..

American has assigned seating but you get more better options if you have Status, or pay more for Main Cabin Extra or a full fare ticket.

Southwest doesn't assign seats but you get a better selection by purchasing Business Select ticket, Early Boarding or a Upgrade at the Gate or by having status. All of the above give you a better place in the boarding order and a better chance to snag one of a few truly better seats. In fact I think Southwest oversells this option, since nothing is guaranteed but around 11% of their passengers buy Early Boarding.

They all monetize seat selection in some way.

I'm perfectly happy to pay Frontiers Low Fare and and addition $8 for a more or less guaranteed aisle seat in the back of the plane.